add order model

db/order.py

 from db.mongo import Model
+from pymongo import ReadPreference
+from bson.objectid import ObjectId
+from pymongo.read_concern import ReadConcern
+from pymongo.write_concern import WriteConcern
 
 
 class Order(Model):
@@ -8,8 +12,40 @@ class Order(Model):
         self.__product_id = product_id
 
     def store(self):
+        client = Model.get_client()
+        my_write_concern_majority = WriteConcern('majority', wtimeout=1000)
+
+        def callback(my_session):
+            product_collection = my_session.client.shoppe.products
+            user_collection = my_session.client.shoppe.users
+
+            # Important:: You must pass the session to the operations.
+            p = product_collection.find_one({'_id': ObjectId(self.__product_id)}, session=my_session)
+            u = user_collection.update_one(
+                {'_id': ObjectId(self.__user_id)},
+                {"$inc": {"balance": -1 * p["price"]}},
+                session=my_session)
+            print(u)
+            print(p)
+            pu = product_collection.update_one(
+                {'_id': ObjectId(self.__product_id)},
+                {"$inc": {"available": -1}},
+                session=my_session)
+
+            print(pu)
+
+        # . Step 2: Start a client session.
+
+        with client.start_session() as session:
+            # Step 3: Use with_transaction to start a transaction, execute the callback, and commit (or abort on error).
+
+            session.with_transaction(callback,
+                                     read_concern=ReadConcern('local'),
+                                     write_concern=my_write_concern_majority,
+                                     read_preference=ReadPreference.PRIMARY)
+
         res = Order.get_collection().insert_one(self.to_dict())
-        return res.inserted_id
+        return str(res.inserted_id)
 
     @staticmethod
     def get_collection():